On a vehicle seat including a seat proper provided with such a structure, a user can adjust the vertical position (the height) of the seat proper relative to the floor, thereby enabling users of different sizes to occupy the seat under conditions that are good for each of them. A seat with a seat proper whose height is adjustable is described, for example, in French Patent Application FR 2 737 868. That seat is provided with four such links coupled together in pairs between two side cheek plates of the seat frame.
Such seats have already been used successfully in the past in various vehicles. In addition, in order to adapt more easily to the space inside vehicles, seat-integrated seatbelt systems have recently been developed that make it easy for a seat containing a seatbelt to be mounted in a vehicle. Since the belt is mounted directly on the seat, it is possible to ignore constraints related to the geometrical shape of the structure of the vehicle that can arise when a seatbelt is to be mounted in the vehicle around the seat as already installed.
An essential condition for implementing such integrated seatbelts is to guarantee safety in the event of frontal impact because the seatbelt is no longer connected to the (rigid) structure of the vehicle itself, but only to the seat. In a seat provided with a seat proper of the above-mentioned type, it was foreseen that such an integrated seatbelt, when driven violently upwards by the occupant in the event of frontal impact, would pivot the links of the seat proper that are used for adjusting the seat vertically. Therefore, a system as described, for example, in DE 100 25 676 was proposed. Unfortunately, such a system is not really effective because its operation relies on lugs having two pivot points, i.e. it is a system that is not only complex, but that also does not guarantee the necessary robustness in the event of impact. An effective system is thus needed in order to guarantee safety for the occupant of such a seat provided with an integrated seatbelt.